Our laboratory's long-range goal is to develop approaches to eradicate HIV. Unlike most viral infections, HIV is able to overcome the host innate and adaptive immune response to establish a life long infection in nearly every infected person. Moreover, standard approaches to develop vaccines and antiviral drugs have not succeeded in providing protection from initial infection or cure. Thus, novel approaches are needed that will more effectively counteract the unique ability of HIV to establish a persistent infection. To achieve this goal, we must first acquire a better understanding of how HIV evades the host innate and adaptive immune responses. We expect that a greater understanding of these viral strategies will inform the design of therapeutic approaches that will result in prevention of infection, long-term remission or cure. As the next step towards this goal, the objective of this application is to understand how the HIV accessory protein, Vpr, allows HIV to evade innate immune responses. Based on strong preliminary data, our central hypothesis is that the innate immune system fails to contain and eradicate HIV because HIV Vpr counteracts host innate immune defense strategies. This hypothesis springs from data demonstrating that Vpr limits the ability of the target macrophage to sense infection and respond to counteract viral spread. In particular, Vpr counteracts a macrophage-specific innate immune response mechanism that recognizes HIV and activates a type I interferon response against virally infected cells. Thus, the rationale of the proposed work is that a greater understanding of how HIV evades the innate immune response will inform approaches that aim to activate the innate immune response to clear infection. By identifying the key viral mechanisms that are needed for persistence, we will better understand which approaches are likely to be effective. Thus, we plan to use the genetic, biochemical and primary cell culture systems we have established to test our central hypothesis and accomplish the objective of this application, which are summarized in following specific aims: 1) Define the restriction factor that limits viral spread in the absence of Vpr; 2) Determine how IFN activates the restriction that is counteracted by Vpr; and 3) Determine how HIV is sensed in the absence of Vpr, which will inform approaches to determine how Vpr limits sensing. At the completion of these studies we expect to establish that Vpr functions to counteract host cell recognition of viral infection by hijacking host DNA repair mechanisms that limit the accumulation of HIV replication intermediates in primary cell culture. Thus, we expect to demonstrate that Vpr prevents the activation of downstream pathways that would otherwise potently inhibit infection. These outcomes are expected to have an important positive impact by identifying the pathways that are most important to target for anti-HIV drug development. Targeting the mechanisms by which the virus disrupts immune pathways will allow the development of safer, more specific treatment approaches that will improve the care of HIV-infected people.